During recent years the question of whether phytochrome regulates certain plant enzyme activities by influencing the rates of enzyme synthesis or by post-translational activation mechanisms has been vigorously debated. There is now good evidence that phytochrome can regulate concentrations of specific messenger RNA populations in some situations. However, the capacity to exert control at a transcriptional level in some systems does not necessarily preclude the possibility that regulation could occur at a post-translational level elsewhere, or even within the same cells. Theoretical considerations apart, the evidence for activation of plant enzymes by phytochrome is not generally strong. Some of the enzymes whose activity is known to be modulated by phytochrome have also been shown to possess post-translational control systems or exist in inactive forms so that the molecular possibilities for such modulation do seem to occur. The lack of direct evidence for the control of such processes by phytochrome may well reflect the technical difficulties involved in this sort of investigation.